Cherry-pick #23886 to 7.x: [Elastic Agent] Fix issues with dynamic inputs and conditions

[blakerouse]

Cherry-pick of PR #23886 to 7.x branch. Original message:

What does this PR do?

Fixes 2 issues that where affecting the usage of dynamic inputs and conditions.

• Loading of the configuration was not excluding the inputs from the go-ucfg variable expansion. Inside of inputs on variable expansion is done by the dynamic inputs code.
• EQL string methods where too strict on type checks, this lowers that to try to convert any type to a string before doing the comparison. This is important in cases where a variable results to an un-expected type, like *eql.null.

Why is it important?

Usage of dynamic inputs work as expected.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [Ingest Manager]: Data Stream not generated for standalone Agent on kibana. #23829
• Closes [Agent] Dynamic inputs fail to get validated #23685

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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• Build Cause: Pull request #24004 opened

• Start Time: 2021-02-11T14:52:03.786+0000

• Duration: 26 min 17 sec

• Commit: d916028

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Total	5838

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Test	Results
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Passed	5814
Skipped	24
Total	5838